Being born either small- or large-for gestational age (SGA or LGA, respectively) is associated with adverse health outcomes in infancy, as well as increased risk of obesity, metabolic and cardiovascular disease later in life. Thus, it is importan to investigate modifiable factors that may alter the fetal environment leading to healthier birth weight (BW) and improved health later in life. Both maternal physical activity (PA) participation and metabolic health during pregnancy are thought to impact fetal development. Maternal PA during pregnancy has been shown to decrease the odds of delivering LGA without increasing odds of SGA. In addition, while maternal lipids increase naturally during pregnancy, high levels are related to increased risk of LGA and low levels are related in increased risk of SGA. Unfortunately, no previous study has concomitantly considered relations among PA during pregnancy, lipid levels and BW, though PA is known to beneficially impact lipids in non-pregnant adults. Additionally, previous reports on relations among PA and BW are based on self-reported measures of PA which are subject to social desirability and recall biases. OBJECTIVE: To determine relations among objectively measured PA and lipids in each trimester of pregnancy and to evaluate associations of both factors with BW. METHODS: We propose to measure lipids and PA among 90 women at 3 time points in pregnancy (i.e., in each trimester) and follow them to birth. English-speaking women with singleton pregnancies < 15 wks. gestation will be recruited at the time of their first prenatal care visit. They will complete a baseline survey, thre venous blood draws (i.e., one in each trimester; two of these are conducted as part of normal medical care), and participate in a telephone interview and accelerometry monitoring around the time of each blood draw. The telephone interviews will use screeners to assess dietary fat intake (an important covariate) and will use the validated Pregnancy PA Questionnaire to assess household, occupational and exercise related PA. Gender and gestational-age specific BW z-scores will be calculated from data abstracted from the birth certificate. Linear regression analyses will examine associations among (1) trimester- specific objective measures of PA and BW z-score, (2) trimester-specific objective measures of PA and maternal lipids, and (3) trimester-specific lipids and BW z-score. We will also consider associations of self- reported PA with both BW z-score and lipids. IMPACT: By utilizing both objectively measured total PA as well as self-reported PA in household, occupational, and exercise domains, results from this study will improve our understanding of the relation between maternal PA and BW. Examining interrelations among PA, lipids and BW may also provide novel insight on a biological mechanism relating different types of PA to improved pregnancy outcomes. This knowledge may translate to the development of more tailored behavioral interventions to improve maternal metabolic health and birth outcomes.